1. Field of the Invention
The present invention relates to a ferroelectric thin film element constructed by forming a ferroelectric thin film on a substrate, for example, to a ferroelectric thin film element suitable for use for a nonvolatile memory, a pyroelectric type infrared sensor and the like.
2. Description of the Prior Art
A ferroelectric thin film element using a ferroelectric material of a lead titanate system or a lead titanate zirconate system is applied to a nonvolatile memory utilizing ferroelectric characteristics, an infrared sensor utilizing pyroelectric characteristics, and the like. This type of ferroelectric thin film element has a structure in which a lower electrode is formed on a substrate, a ferroelectric thin film is formed on the lower electrode by a thin film forming process such as sputtering, and an upper electrode is further formed on the ferroelectric thin film.
Meanwhile, in order to sufficiently exhibit the material characteristics of the ferroelectric thin film, it is necessary to make the crystal orientation of the ferroelectric thin film equal to the direction of an easy polarization axis, that is, a spontaneous polarization axis of the thin film. When the crystal orientation of the ferroelectric thin film is made equal to the direction of the spontaneous polarization axis of the thin film, it is possible to, for example, reduce the volume change of the thin film in the case of applying alternating pulse cycles to the polarization, thereby to make it possible to prevent the fatigue of the material characteristics.
One example of the above described ferroelectric thin film element is disclosed in Japanese Patent Laid-Open Gazette No. 162369/1987. This document discloses a structure in which a MgO thin film oriented in the direction (100) is formed on a silicon substrate oriented in the direction (100), a lower electrode composed of Pt is formed on the MgO thin film, and a PbTiO.sub.3 thin film having a spontaneous polarization axis in the direction of a c axis, that is, in the direction (001) is formed on the lower electrode.
In order to exhibit the characteristics of the ferroelectric thin film, the ferroelectric thin film must have a crystalline structure of a perovskite phase. Consequently, in forming the ferroelectric thin film, the temperature of the thin film must be increased to temperatures in excess of the crystallizing temperature of the thin film, that is, approximately 500.degree. C. In this case, the lower electrode formed beneath the ferroelectric thin film is similarly heated. Consequently, a material superior in high-temperature stability is required as a material composing an electrode used in the dielectric thin film. Conventionally, an electrode composed of Pt has been widely used.
The electrode composed of Pt is superior in oxidation resistance under high temperatures and is low in reactivity with the ferroelectric thin film. When a structure in which the ferroelectric thin film is interposed between upper and lower electrodes composed of Pt is formed, however, a short is liable to occur between the upper and lower electrodes.
In order to solve the above described problem of the electrode composed of Pt, an electrode composed of an alloy thin film of Ni--Cr--Al alloy or an alloy thin film of Ni--Al alloy has been proposed (Japanese Patent Laid-Open Gazette No. 276615/1991). When the electrode composed of the above described alloy thin film is formed on a silicon substrate, the electrode reacts with the silicon substrate at a temperature at which a ferroelectric thin film is formed, to form a Ni--Cr--Al--Si compound or a Ni--Al--Si compound. Accordingly, the electrode composed of the alloy thin film may, in some cases, fail to function as an electrode.
Furthermore, in order to prevent the reaction between the electrode and the silicon substrate, it is considered that an amorphous SiO.sub.2 layer commonly used as a surface oxidized layer is formed on the surface of the silicon substrate. In this case, the electrode does not react with the amorphous SiO.sub.2 layer. However, the electrode is formed on the amorphous SiO.sub.2 layer. Accordingly, an alloy of Ni--Cr--Al alloy or an alloy of Ni--Al alloy becomes a crystal which is not oriented. As a result, when a ferroelectric thin film is formed on the electrode composed of the alloy thin film of Ni--Cr--Al alloy or the alloy thin film of Ni--Al alloy, the ferroelectric thin film cannot have superior crystallizability. Specifically, the crystal orientation of the ferroelectric thin film cannot be made equal to the direction of the spontaneous polarization axis of the thin film, so that the ferroelectric thin film cannot exhibit sufficient characteristics as a ferroelectric thin film element.